Skeletal Systembio

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5 ARTS

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The Skeletal System serves many importantfunctions; it provides the shape and form forour bodies in addition to supporting,protecting, allowing bodily movement,producing blood for the body, and storingminerals.

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The ability to move is named among thecharacteristics of living organisms.

There are two (2) types of movements seen inliving organisms:- Slight movements of parts of the body

- Movement of the entire body.

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Examples, plants move their roots in thedirection of moisture and the Amoeba whichis found in water, and is unicellular (thewhole organism is a single cell).

Like other living organisms move, but unlikehumans they move only in response to the

presence of food particles.

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Their movement is achieved by projecting parts

of its cell membrane (pseudopodia- false feet)ahead of another part (like crawling in a bag).

On the other hand, humans are multi-cellular

(consisting of 60 billion cells) and more complexorganisms that carry out movement for variousreasons and are more organized.

Man consists of a framework of bones thatsupports and gives form to the body. Thisframework is called a skeleton.

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The Skeleton is divided into two sections: the Axial Skeleton- which is comprised of the

cranium / skull, sternum/, ribcage and vertebralColumn.

Appendicular Skeleton- which includes the limbs,the pectoral girdle (shoulder bones) and pelvicgirdle.

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Its 206 bones form a rigid framework towhich the softer tissues and organs of thebody are attached. Protection

Vital organs are protected by the skeletalsystem.

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In multi-cellular organisms like man, theskeleton provides a mechanism of support asit provides a frame for the body (to hang on),example the vertebral column.

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The brain is protected by the surroundingskull as the heart and lungs are encased bythe sternum and rib cage. Bodily movement is carried out by theinteraction of the muscular and skeletalsystems. For this reason, they are often groupedtogether as the musculo-skeletal system.

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Muscles are connected to bones by tendons. Bones are connected to each other by ligaments. Where bones meet one another is typically called ajoint. Muscles which cause movement of a joint areconnected to two different bones and contract topull them together.

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An example would be the contraction of thebiceps and a relaxation of the triceps. This produces a bend at the elbow. The contraction of the triceps andrelaxation of the biceps produces the effect

of straightening the arm.

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Blood cells are produced by the marrowlocated in some bones. An average of 2.6 million red blood cells areproduced each second by the bone marrow toreplace those worn out and destroyed by theliver.

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Bones serve as a storage area for mineralssuch as calcium and phosphorus. When an excess is present in the blood,buildup will occur within the bones.

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When the supply of these minerals within theblood is low, it will be withdrawn from thebones to replenish the supply.

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The human skeleton is divided into twodistinct parts: The axial skeleton consists of bones that formthe axis of the body and support and protect the

organs of the head, neck, and trunk. The Skull The Sternum The Ribs The Vertebral Column

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The parts of the body are bound together,and supported, by connective tissue.

Blood, which has been discussed incirculation, is a connective tissue.

Some of the other types of connective

tissue will be discussed in this presentation.

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They are fibrous connective tissues; cartilage;and bone.

Connective tissues contain living cells whichsecrete chemical substances to be depositedbetween the cells in order to strengthen the

tissue.

These secretions form an intercellular matrix.

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The cells secrete collagen, a protein whichforms strong fibres.

Examples of this are the ligaments which holdjoints in position and the tendons whichattach muscles to bones.

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This is an elastic, smooth, shiny form ofconnective tissue.

Cartilage cells secrete a hard form of collagenand the matrix pushes apart the living cells.

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The skeleton is first laid down in the foetusas cartilage which later develops into bone.

Other types of cartilage remain as suchthroughout adult life.

Fig. 1 shows cartilage tissue. Cartilage is

elastic, but does not stretch or bend easily.

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Cartilage which develops into bone has a primarycentre at which bone cells are formed.

The bone cells deposit the bone matrix, mademainly of calcium phosphate.

This centre is a centre ofossification (i.e. boneformation).

As ossification spreads, the cartilage is absorbed

and the cartilage cells die.

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Ossification spreads along the bone, and ahard, rigid matrix is formed with bone cellsinside it.

The development of ossification in youngbones is shown in Fig. 2.

The bone cells in bones are scattered inconcentric circles round canals (Haversiancanals) which carry fine blood vessels andnerves.

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Fig. 3 shows the living cells embedded in thehard matrix.

Fig. 4 shows the manner in which the cellsare gradually forced away from the canals,although they still obtain nourishment from

the blood supply.

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Each cell has many fine, fibrous processeswhich form before the matrix is deposited;these are shown as lines radiating.

In human beings, the long bones grow ateach end, during childhood and adolescence(growth stops at an age of 20 approximately).

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The bone becomes large, and at the sametime it thickens on the outside.

The" inside of the bone contains spongybone, in which cavities are left as cartilagedies away; the outside consists of hard bonewith few such cavities.

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The interior of the bone is hollow and thesize of the interior' increases as the bonethickens.

Fig. 5 shows a diagram of a long bone.

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The ends of the bone consist of spongy bonefilled with red marrow in the bone cavities.

Erythrocytes and granulocyte leucocytes aremanufactured in this marrow.

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The hollow centre of the bone contains yellowbone marrow which mainly consists of fatcells.

Broken bones are knitted together by theoutside layer of bone secreting fresh bonematrix.

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This is also a form of connective tissue,although it is not used for support.

Enlarged cells are filled with deposits of oil inthe cell vacuoles; the oil gradually pushesaside the cytoplasm until the cell is practicallyfilled with fat.

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An axial skeleton, consisting of the backboneand skull, forms the foundation of the humanskeleton.

Attached to it is a rib cage.

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The appendicular skeleton consists of thefour limbs attached to two bony girdles.

Fig. 6 shows the skeleton of a man.

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The backbone is the central axis of the body;it consists of 33 separate bones firmlyconnected to each other, yet allowing alimited amount of movement on each other

(resulting in the flexibility of the backbone).

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It extends the length of the trunk, and theseparate bones are bound together byligaments.

The backbone has to support the weight ofthe trunk, and is curved in an S-shape for thispurpose. (See Fig. 7).

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Each bone is called a vertebra, and althoughthe vertebrae all have the same basic plan,the actual shape of any one bone varieswith its position in the backbone.

The five regions of the backbone, and thenumber of vertebrae in each, are shown in

Fig. 7.

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The cervical region is in the neck; thethoracic region is behind the chest, and theribs are attached to these vertebrae; thelumbar region is behind the abdomen, andthese vertebrae protect the abdomen.

The sacral region consists of five vertebraefused together, and the pelvic girdle isattached to this portion of the backbone.

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The coccyx consists of four small, fusedvertebrae, and it is the remnant of a tail.

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The ventral (front) part of the bone is athick, protective mass of bone, the centrum. Figs. 8and 10 show the basic structure of a

vertebra.

The centrum forms the main support for thebackbone; attached to it dorsally is theneural arch, a ring of bone through whichpasses the spinal cord.

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The neural arches of all the vertebrae forma bony tube for the protection of the spinalcord.

Transverse processes project on either sideof the neural arch.

Between the transverse process and the

centrum is a hole, a foramen, on each sideof the vertebra.

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The foramen provide exits for the spinalnerves.

The neural spine is a long bony process onthe dorsal (back) aspect of the neural arch.

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Four facets provide surfaces for articulation,that is, surfaces on which the vertebrae arecapable of restricted movement, allowing thebackbone to bend; the shape of the surfaces

controls and restricts the movement of onebone on another.

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Two of the facets are on the top of, and twoare on the bottom of, each vertebra, situatedat the point where the transverse processes

join the neural arch.

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Muscles and ligaments are attached to thetransverse processes and the neural spine,binding the vertebrae together andcontrolling the movement of the backbone.

Discs of cartilage between the centra of thevertebrae absorb shock.

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These vertebrae are recognisable by thepresence ofvertebrarterial canals for thevertebral arteries on either side of thecentrum.

The neural spine is forked at the end.

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The first two vertebrae have a specialstructure, due to their function; the first isthe atlas and the second is the axis.

Fig. 11 shows a diagram of a cervicalvertebra.

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This bone is shown in Fig. 11.

It has no centrum. Its superior facets arelarge and articulate with the skull, allowing a

rocking movement.

There are no transverse processes and no

neural spine.

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The centrum of the axis has a strong tooth-like process, the odontoid peg, which fitsinto a hole in the atlas.

This allows a turning movement in whichthe atlas moves with the skull.

The atlas and axis together allow movement

of the skull in all directions. (See Fig. 11).

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These possess very long neural spines whichare bound together by ligaments.

There are two long transverse processes with

facets on top fitting into facets on the bottomof the vertebra above.

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Additional facets on either side of thecentrum, and at the ends of the transverse

processes, are provided for articulation withthe ribs; each thoracic vertebra thus haseight facets.

Muscles are attached to the neural spineand the transverse processes.

Notice the overlap of the neural spines andthe transverse processes in this region ofthe backbone. (See Fig. 12).

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These are very massive bones as theyprovide the only support for the trunk inthe abdominal region.

They possess large broad transverseprocesses and a short broad neural spine.

The superior and inferior facets are both

large. (See Fig. 13).

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These are fused together, forming thesacrum, which is the base for the pelvis.

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The skull consists of the cranium and the facebones. The cranium is formed from many bones

joined together by interlocking, serratededges, which become fused in adulthood.

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The cranium encloses the brain and protectsit.

Entrances to the cranium are provided by theeye sockets, the nasal passages, and theforamen magnum, which is the entrance for

the spinal cord to the brain.

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Two round swellings at the base of the skullon either side of the foramen magnum reston the facets of the atlas.

The upper jawbone is fused to the base ofthe cranium; the lower jaw bone is hinged tothe temporal bone of the cranium.

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The nose has a bony framework in the upperpart and a framework of cartilage in the lower

part.

The cheek boneis a bony process of theupper jaw bone. (See Figs. 14 and 15).

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These are twelve pairs of ribs in the skeleton,and all articulate with the backbone.

The upper seven are joined directly to thesternum (or breast bone) by cartilage at theend of the rib.

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The next three are attached to the rib aboveby cartilage, while the bottom two ribs arenot connected to the sternum or to the ribabove; these are called floatingribs.

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Fig. 16 is a diagram of the rib cage.

Fig. 17 shows the structure of a rib; it is acurved, flat bone with a head, and a tubercle,

a projecting process near the head on theoutside of the rib.

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There is a facet on the head and one on thetubercle.

The facet on the head articulates with facets

on the centra of two vertebrae.

The tubercle articulates on the facet of a

transverse process.

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The rib cage gives rigidity to the pectoralgirdle and protects the vital organs in thethoracic cavity.

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This consists of two arms articulating withthe pectoral girdle and two legs articulatingwith the pelvic girdle.

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This girdle contains two bones, thescapulae, (single scapula), or shoulderblades, and two others, the clavicles, orcollar bones.

The scapula is a flat triangular-shaped bonewith a spine projecting from it. (See Fig.18).

It is embedded in muscle and is dorsal tothe rib cage.

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At the head of the bone is a socket for thehumerus.

A facet on the spine of the scapula provides

an articulatory surface for the clavicle.

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The clavicle is a long flat, gently curvedbone. It articulates with the scapula at oneend, and with the sternum, or breast bone,at the other end. (See Fig. 19).

There is no complete bony girdle structure;the girdle is formed by the clavicle, the

scapula, and the strong muscles attached tothe backbone.

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Fig. 20 shows the pectoral girdle in relationto the ribs.

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The bone in the upper arm is the humerus,which lies between the shoulder joint and theelbow joint.

The radius and the ulna are situated in theforearm; the ulna articulates with thehumerus to form the elbow joint.

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The radius articulates with both thehumerus and the ulna and it is capable ofrotation.

The radius rotates from above the ulna tobelow it when the hand is turned over.

The hand has two rows of small carpalbones in the wrist.

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The metacarpalbones form the palm of thehand.

The phalangesform the digits, with three

phalanges in each finger, and two in thethumb. (See Fig. 21).

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The girdle consists of a bowl-shaped, solidmass of bone formed from three fused bones.

The dorsal part is firmly fixed to the sacrum,

and the thinner, ventral portion is fused inthe middle, as shown in Fig. 22.

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A socket in the pelvis takes the ball shaped


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A socket in the pelvis takes the ball-shapedhead of the femur bone of the leg to form the

hip joint.

Fig. 23 shows the socket, and the femur; thetwo bones on the left in the figure are the

ventral part of the girdle, and they are fusedwith two similar bones on the other side ofthe body.

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The pelvic girdle is a strong, bony structure,suitable for taking the weight of the trunkand transmitting it to the legs.

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The skeleton gives support and shape to thebody.

It allows movement of the body through

articulation of bones at joints by the action ofmuscles on bones.

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It gives a protective covering, the bestprotection given by the body, to the mostvital organs.

Fig. 6 shows the skeleton, and the particulararrangement of bones, discussed above, canbe seen in relation to the whole skeleton.

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Another function of the skeleton is that itprovides a store of calcium from whichcalcium ions may be moved into the blood asrequired.

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Two, or more, bones, are connected togetherby ligaments, which form a fibrous capsulesurrounding the joint.

Smooth, articular cartilage on the ends of thebones facilitates the movement of one bonerelative to the other, and also absorbs shock.

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The space between the two pads ofcartilage is filled with synovial fluid, a liquidwhich lubricates and cushions the joint.

The fluid is contained in a. delicatemembrane, the synovial membrane, whichitself is contained in the fibrous capsulesurrounding the joint.

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A diagram of a cross-section through atypical joint is shown in Fig. 26, andillustrates the parts of the joint describedabove.

Damage to the joint causes excess synovialfluid to be formed, and the fibrous capsulebulges, causing the joint to swell.

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For example, water on the knee, and "tenniselbow", are complaints of this type.

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Ajoint is described according to the degreeof movement it permits.

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The main types are: ball and socket joints,


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ypwhich allow free movement in all directions;hinge joints, which allow movement in oneplane only; gliding joints, in which two bonesurfaces move over each other, e.g. thecarpals and the tarsals; fixed joints, e.g. thesutures, which join the bones of the cranium.

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An example of this type is the hip joint,shown in Fig. 23, and in diagram form inFig. 27.

In the hip joint, the head of the femur isball-shaped and fits into a socket in thepelvis.

The ball in the socket allows free rotation inall directions.

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Ligaments bind the femur to the pelvis, andthe manner in which they are attached to the

bone is shown in Fig. 28.

These ligaments form the fibrous capsuleenclosing the joint; the remaining structuresof synovial fluid and the synovial membraneare as described above under the structure ofa joint.

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Another example of a ball and socket joint isthe shoulder joint; a ball-shaped head in thehumerus fits into a socket in the scapula.

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In this type of joint, the rounded end of onebone fits into the hollow of a second bone;the two structures are flat in one plane,allowing movement in one direction only.

A diagram of a section through a hinge jointis given in Fig. 29.

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The knee joint (see Fig. 30) is an example of ahinge joint; the round-shaped lower end of thefemur articulates on the flattened surface of thetibia.

Ligaments bind the bones together, forming afibrous capsule.

The patella protects the blood vessels and nervespassing the joint, as the flesh is thin at this point.

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The elbow joint is another example of a


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hinge joint; in it, the rounded lower end ofthe humerus fits into a hollow of the ulna.

The ligaments binding the humerus to theulna are shown in Fig. 31.

The radius is also bound to the ulna andhumerus by ligaments, which firmly anchorthe head of the radius to the ulna and tothe humerus.

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These ligaments enclose two fibrous capsules


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gfor the two articulatory surfaces.

The olecranon process, a projection on theulna, prevents movement backwards past thestraightened position.

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Most joints permit bones to be moved relativeto each other, and thus allow movement andlocomotion of the body.

The bone surfaces in a joint are preventedfrom wearing by the synovial fluid and thecartilage at the end of the bones.

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The ligaments prevent dislocation of thejoint, that is, the head of one bone beingremoved from the socket of another.

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Voluntary muscles are attached to bones toprovide movement of joints.

They are therefore sometimes called

skeletal muscles.

Their appearance under the microscope isthat of striped, or striated, tissue, hence

voluntary muscle is called striped muscle.

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Involuntary muscles mostly surround the


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viscera; their microscopic appearance is thatof smooth, or un-striated, tissue.

Involuntary muscle is called smooth muscle,or plain muscle.

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Heart muscle is a special type of tissue and isdescribed below; it is also called cardiacmuscle. Figs. 32, 33, and 34 are photomicrographs of

the three types of muscle.

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The bulk of the body of vertebrates, includingman, is muscle.

The lean meat from domestic, and other,

animals used for food is voluntary muscle.

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Under a microscope, the muscle is seen to becomposed of long fibres which are cylindrical


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composed of long fibres, which are cylindricalin cross-section and covered with a thin

membrane.

Each fibre contains several nuclei, as, during

growth, the nucleus of a muscle cellundergoes division, without division of thecytoplasm.

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Each fibre consists of cytoplasm and longfibrils;the fibrils are striated, i.e. they havealternate bands of light and dark colouredprotein, and it is their structure which gives

the muscle its striped appearance.

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The structure of a fibre is showndiagrammatically in Fig 35 A cross section


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diagrammatically in Fig. 35. A cross-sectionof a voluntary muscle is shown in Fig. 36.

Bundles of fibres are enclosed in amembranous sheath; a muscle is composedof many such bundles.

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A network of capillaries and nerves passesbetween the fibres with nerve endings


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between the fibres, with nerve endings,effectors, attached to the fibres.

A tough, shining, white sheath of connectivetissue encloses the muscle, and continues

from the end of the muscle to form a tendon.

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Each muscle fibre has several motor neuroneffectorsattached to its sheath.

An impulse, conducted by the motor nerve,

causes contraction of the muscle fibre in avery short period of time (about 0-01seconds).

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On contraction of the fibres, the muscle fibrebecomes shorter and thicker, but its total


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becomes shorter and thicker, but its totalvolume remains the same.

Each muscle fibre behaves according to the"all-or-none" principle; if the nervousimpulse is strong enough to causecontraction, then it causes completecontraction of the fibre.

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The muscular effort is determined by thenumber of fibres contracted by nervousi l


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impulses.

A single impulse contracts a muscle fibre forabout 0.04 seconds, after that it relaxesagain.

For a sustained contraction, a series ofnervous impulses is sent, so that theindividual contractions merge into onesustained contraction.

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Sufficient impulses are conducted by motorneurons to contract a sufficient number of


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neurons to contract a sufficient number offibres to produce the necessary muscular

effort.

Most voluntary muscles are controlled by the

motor centres of the brain, and so voluntarymuscular action is under the control of thewill.

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Voluntary muscles contain protein, glycogenand mineral salts; the glycogen is stored


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g y gready to provide energy for muscular action.

Tissue respiration releases heat and energyfor muscular action and forms wasteproducts.

Muscles become fatigued with prolongeduse, and this is explained under muscularfatigue.

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This type of muscle tissue contains spindle-shaped cells, each cell containing one nucleusin the centre of the cell. (See Fig. 17.37).

The cells are packed-tightly together,forming sheetsof muscle tissue.

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A sheet of involuntary muscle usually consistsof two layers with the axes of the muscle


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of two layers, with the axes of the musclecells in each layer at right angles to each

other.

The sheets of muscle mainly envelop visceral

organs, e.g. the intestine; the bladder.

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Motor neurons of the autonomic system haveeffector endings on the sheet of muscle;


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effectorendings on the sheet of muscle;impulses transmitted by these nerves controlthe waves of contraction in the sheet ofmuscle.

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A very slow con traction, time of about 15seconds is characteristic of this type ofmuscle.

Each part of the muscle contracts and relaxesin this time, and the wave of contraction andrelaxation passes along the sheet of muscle.

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No fatigue is experienced due to the slow rateof contraction, and the supply of materialsfrom the blood is adequate to meet the needsof tissue respiration in the muscle.

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Heart muscle consists of a network ofstripped fibres which branch, and join otherfibres.

There is no membrane surrounding thefibre as in voluntary muscles.

The fibres contain separate cells with anucleus in each.

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This type of muscle is like striped muscle (asthe fibres contain striated fibrils), and likesmooth muscle (as the cells are uninucleate).(See Fig. 38).

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The action of heart muscle differs from both

voluntary and involuntary muscles in being bothautomatic and rhythmic.

The contraction time is faster than that of

involuntary muscle but slower than that ofvoluntary muscle.

It is not under the control of the motor centres ofthe brain, but it is controlled by the autonomic

nervous system.

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It acts independently of neuron connections,and beats (contracts) rhythmically andcontinually.

It is supplied with autonomic nerve endingswhich influence the rateof beating.

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The contractions of muscles cause themovement of joints and of viscera.

Muscles also give rigidity to the skeleton by

preventing the movement of bones.

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Tissue respiration releases heat to warm thebody and energy to produce work bymuscular action.

Certain muscles provide protection for partsof the body, e.g. the abdominal musclesprotect the viscera.

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All voluntary muscles work in pairs inopposition to each other.

This is necessary because a muscle can onlycontract or relax, so that it can only pull

and cannot push.

The muscles that move a joint either causethe angle of a joint to decrease (flexors)orto increase [extensors).

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For example, in the elbow joint (see Fig. 39),the biceps muscle is a flexor; it is situated infront of the humerus and is attached by twotendons to its origin on the scapula.

Its insertion is on the radius, where it isattached by one tendon.

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Contraction of the biceps raises the forearm,decreasing the angle of the joint.


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The triceps muscle is situated behind thehumerus; it is attached by threetendons toits origins, one to the scapula and two to thehumerus.

Its insertion is on the ulna, where it isattached by one tendon.

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The triceps is an extensor, and increases theangle of the joint.


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The triceps muscle steadies the arm, giving itrigidity when the biceps muscle is used to lifta load.

The brain coordinates the muscularmovement by sending nervous impulseswhich are sufficient to cause sufficientmuscular action to lift the load.

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Fig. 40 shows diagrammatically the pair ofmuscles in opposition to each other and the

ti f th bi l i lifti l d


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action of the biceps muscle in lifting a load.

The brain, from information received fromthe eye, and from previous learnedexperience, estimates the weight of the load,

and causes muscular action which suppliesjust sufficient effort to move the estimatedweight.

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Posture is the manner in which the body isheld at rest by the muscles attached to theskeleton.

In good posture, the backbone is upright,with its normal curvatures, i.e. the cervicalregion is convex forwards, the thoracicregion is concave forwards, the lumbar

region is convex forwards, and the sacralregion is concave forwards.

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The weight of the body is balanced on thefeet through the pelvis.


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In bad posture, the point of balance is upset,causing a strain on the muscles tocompensate for the lack of proper balance;this produces uneven development of the

muscles and a misshapen body.

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If the backbone is curved too much, it crampsthe thoracic cavity 'and hinders proper


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breathing; is bad for all activities since

insufficient oxygen is made available.

The abdominal cavity can also be cramped,and this hinders peristalsis.

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Good posture is a matter of habit.

O th h bit i l t it i diffi lt t i


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Once the habit is lost, it is difficult to regain.

It is important that children learn goodposture for standing; for writing, and forreading.

Good and bad postures are shown in Fig. 40.

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1. What are the structures and properties ofthe different types of connective tissue?

2. How do bones grow? Illustrate youranswer with particular reference to thehealing ofa broken long bone.

3. Make a labelled diagram of the basic planof a vertebra.

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4. Describe the mode of articulation of the skullon the backbone.


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5. Compare the joints of the leg with those ofthe arm.

6. Using the elbow as an example of a joint,explain how muscles cause movement of thejoint.

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7. Describe the structure of a voluntarymuscle. What is the source of energy forthe movement of the muscle?


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8. Describe, with a suitable example, thestructure and function of involuntary, orsmooth muscle.

9. Write a brief account of good posture andits importance.

10. A wound is made in an arm of a man.Describe all the defence mechanisms of

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